全新java初学者实践教程10(java SE5.0版)
DX l3 jdk5的集合类
w,> ceu/ 2Kovvh y# YvL5>; >VM@9Cph 上次课我们学过了数组,知道它只是一组数(或是对象),但是有些自己的特性。在[font="Times]java里还有一类东西与数组类似,也是有着特性的一组数[font="Times](或是对象[font="Times]),叫做集合类。
"VR>nyG% 我们上节课讲到了,数组的长度在创建时已经确定了,但是有时候我们事先根本不知道长度是多少啊,比如我们做电子商务网站时,有个购物车程序。你总不能用数组规定,人家只能买[font="Times]5样东西吧。你就是把长度定为[font="Times]10000也不行,万一遇上个特别有钱的呢!呵呵,这只是开玩笑的。我们会使用集合类解决这个问题。
3#`_t :"A [font="Times] 集合类是放在[font="Times]java.util.*;这个包里。集合类存放的都是对象的引用,而非对象本身,为了说起来方便些,我们称集合中的对象就是指集合中对象的引用([font="Times]reference)。引用的概念大家不会忘了吧,在前边我们讲
数据类型时讲的。
A}?n.MAX> zs:OHEZw [font="Times] 集合类型主要有[font="Times]3种:[font="Times]set(集)、[font="Times]list(列表)、[font="Times]map(映射[font="Times])和[font="Times]Queue(队列)。[font="Times]//队列为[font="Times]jdk5中的加上的[font="Times]
Y^Y|\0 ?8X;F"Ba [font="Times](1) Set
NK;%c-r0v7 集([font="Times]set)是最简单的一种集合,它的对象不按特定方式排序,只是简单的把对象加入集合中,就像往口袋里放东西。对集中成员的访问和操作是通过集中对象的引用进行的,所以集中不能有重复对象。我们知道数学上的集合也是[font="Times]Set这个,集合里面一定是没有重复的元素的。
b1JXC=*@ p;zV4uSv ([font="Times]2)[font="Times]List
0eUK' 列表([font="Times]List)的主要特征是其对象以线性方式存储,没有特定顺序,只有一个开头和一个结尾,当然,它与根本没有顺序的[font="Times]Set是不同的。它是链表嘛,一条链肯定有顺序这个顺序就不一定了。
FJomUVR . rg64f'+Eug X*hY?'Rp [font="Times] ([font="Times]3)[font="Times]Map
fT.18{'> [font="Times] 映射([font="Times]Map),这个在[font="Times]java里不是地图的意思,其实地图也是映射哈。它里面的东西是键-值对([font="Times]key-value)出现的,键值对是什么呢?举个例子,比如我们查字典,用部首查字法。目录那个字就是键,这个字的解释就是值。键和值成对出现。这样说可以理解吧。这也是很常用的数据结构哦。
GM=r{F
& *UhYX)J uOUgU$%zqH [font="Times] ([font="Times]4)[font="Times]Queue
UJMM& [font="Times] 在[font="Times]jdk5.0以前,通常的实现方式是使用[font="Times]java.util.List集合来模仿[font="Times]Queue。[font="Times]Queue的概念通过把对象添加(称为[font="Times]enqueuing的操作)到[font="Times]List的尾部(即[font="Times]Queue的后部)并通过从[font="Times]List的头部(即[font="Times]Queue的前部)提取对象而从[font="Times] List中移除(称为[font="Times]dequeuing的操作)来模拟。你需要执行先进先出的动作时可以直接使用[font="Times]Queue接口就可以了。
9z6-HZG'~< [font="Times] 这[font="Times]4个东西,有时候功能还不太完善,需要有些子类继承它的特性。[font="Times]Set的子接口有[font="Times]TreeSet,SortedSet,[font="Times]List的有[font="Times]ArrayList等,[font="Times]Map里有[font="Times]HashMap,HashTable等,[font="Times]Queue里面有[font="Times]BlockingQueue等。我们来看看例子吧:
g|ewc'y jI%v[]V ?~c=Sa- 实践:[font="Times] Set举例
`dekaRo [font="Times] import java.util.*;
smaPZ^;; j [font="Times]public class SetExample {
i#]}k [font="Times] public static void main(String[] args) {
PKFjM~J [font="Times] Set set = new HashSet(); //HashSet是[font="Times]Set的子接口
>uT,Z,7O [font="Times] set.add("one");
2%P{fJbwd [font="Times] set.add("second");
A?V}$PTlx [font="Times] set.add("3rd");
`,a6su (? [font="Times] set.add(new Integer(4));
S^(OjS [font="Times] set.add(new Float( 5.0F ));
Q46^i7= [font="Times] set.add("second");
'ol8l Ia.P [font="Times] set.add(new Integer(4));
?!TFoD2' [font="Times] System.out.println(set);
{~q"Y]? [font="Times] }}
`u6CuH5 MIma:N_c [font="Times]List举例:
Y1fcp_]m [font="Times] import java.util.*;
zGgPW [font="Times]public class ListExample {
-!i1xR(;h [font="Times] public static void main(String[] args) {
HR'sMu3 [font="Times] List list = new ArrayList();
Pt< JF [font="Times] list.add("one");
0%J0.USkM7 [font="Times] list.add("second");
9/2VU<
K [font="Times] list.add("3rd");
AB(WK9o [font="Times] list.add(new Integer(4));
=2v/f_ [font="Times] list.add(new Float( 5.0F ));
z7TMg^9# [font="Times] list.add("second");
l@:Tw.+/9 [font="Times] list.add(new Integer(4));
E$l 4v>iA [font="Times] System.out.println(list);
#C^)W/dP [font="Times] }}
@A32|p} fk%W07x! mD@*vq [font="Times]Map举例
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? [font="Times]import java.util.Map;
J]G]
<) [font="Times]import java.util.HashMap;
I<E~= [font="Times]import java.util.Iterator;
;IyA"C(i [font="Times]import java.io.FileReader;
DR]=\HQ [font="Times]
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^ [font="Times]public class MapExample {
iWu [font="Times] public static void main(String[] args) throws java.io.FileNotFoundException {
>s dT=6v [font="Times] Map word_count_map = new HashMap();
Gbj^o o [font="Times] FileReader reader = new FileReader(args[0]);
F(h
jP [font="Times] Iterator words = new WordStreamIterator(reader);
(4]M7b[S$ [font="Times]
:Kq]b@X [font="Times] while ( words.hasNext() ) {
9r2l~zE [font="Times] String word = (String) words.next();
RvQa&r5l [font="Times] String word_lowercase = word.toLowerCase();
zm>^!j
! [font="Times] Integer frequency = (Integer)word_count_map.get(word_lowercase);
Y oDL/ [font="Times]if ( frequency == null ) {
phCItN; [font="Times] frequency = new Integer(1);
EKu%I~eM [font="Times] } else {
ZZ'5BfI"I% [font="Times] int value = frequency.intValue();
lo!^h]iE ! [font="Times] frequency = new Integer(value + 1);}
P^'TI[\L9 [font="Times] word_count_map.put(word_lowercase, frequency);
:/A7Z<u, [font="Times] }
tzIcR
#Z [font="Times] System.out.println(word_count_map);
CghlyT [font="Times] }}
z|Y Ms? P{m(.EC_ {$>Pg/ [font="Times]Queue举例:
_c!$K#Yl{ [font="Times]import java.io.IOException;
xP{)+$n [font="Times]import java.io.PrintStream;
t;HM [font="Times]import java.util.LinkedList;
@T.+:U@S [font="Times]import java.util.Queue;
J2ZV\8t [font="Times]
ohU}ST:9 [font="Times]public class QueueTester {
^cDHC^Wm [font="Times] public Queue q; //发现了一个奇怪的语法,这个尖括号是泛型声明
j_3`J8WwF [font="Times] public QueueTester() {q = new LinkedList();}
hs^K9Jt [font="Times]public void testFIFO(PrintStream out) throws IOException {
S:(YZ%# [font="Times] q.add("First");
"ov270: [font="Times] q.add("Second");
k\76`!B [font="Times] q.add("Third");
}G/!9Zq [font="Times] Object o;
*Jwx,wF}4 [font="Times] while ((o = q.poll()) != null) {
ldFR%v>9 [font="Times] out.println(o);}}
;yu#Bs [font="Times] public static void main(String[] args) {
0u) m9eg [font="Times] QueueTester tester = new QueueTester();
/7N&4FrG [font="Times] try { tester.testFIFO(System.out);
)L/0X40<. [font="Times] } catch (IOException e) {
;kDUQw [font="Times] e.printStackTrace(); } }}
.Gt_~x n58yR -" fI
v?HD:j 上述例子和一些相关例子(共6个例子)打包下载[font="Times]
<N{wFvF 总结:
XCyU)[wY 刚才我们看了上述例子了,对集合类有了一个初步的认识,它们跟数组的区别不只是长度问题,在集合类里面放进去的类型可以是任意的。不像数组,数组里面的类型是一样的。这样的话,对于集合类来说即使好事,也是坏事。因为你不考虑类型可以随意的放,但是你放进去什么就不知道了不容易找。
d/Sw.=vq 还有啊,什么叫泛型声明啊?我们下次课就告诉你。这可是[font="Times]jdk5的酷炫之处哦。